Changes in the direction of drilling of bore holes are required for a number of reasons. The most frequent reason is to change from vertical drilling to horizontal drilling or drilling at any particular angle other than vertical. Horizontal drilling has been known for many years and there are a number of established methods of changing the direction from vertical drilling to horizontal drilling. For example long radius drilling which is used for accessing oil reservoirs in remote locations, under cities, offshore or to avoid geological isolation.
Medium radius drilling is used for pinnacle relief, fractured formations and gas and water coning. Short radius drilling can be used for all these applications. The particular method used is chosen based on the economic considerations of the particular well.
The most common existing method to change the direction of drilling is to use a bent support for the drill bit or a "bent sub" as it is often referred to. Typically a drill bit is used which is powered by a motor and the bent sub is positioned behind the motor. It is also possible for the bent sub to be positioned in front of the motor. The bent sub effectively causes the axis of rotation of the drill be to be at a different angle to that of the drill pipe. Continuous drilling with the bent sub causes continuous changes of direction which results in a curved well hole in the direction of the bend of the bent sub. When the required curvature has been achieved drilling can be stopped and the bent sub changed for a straight sub to resume straight drilling.
Alternatively, the entire drill pipe can be rotated at the surface resulting in a small rotation of the bent sub, motor and drill bit assembly. The bend of the bent sub is now positioned in a different direction and drilling can be resumed in this different direction.
Positional sensors such as gyroscopic sensors are used to check the progress and direction of the drilling to establish what adjustments to the drilling angle are required.
A disadvantage of this existing method of directional drilling is that the drilling tool has to be removed from the bore hole and changed before drilling in the straight direction can be recommenced. This results in an expensive operation and increases the time to complete the required drilling.
A further disadvantage is that when drilling is restarted in a new direction it is often the case that the drill bit kicks in an unpredictable direction due to unevenness in the hardness of the formation at the point of stoppage of the drill head.
A further disadvantage with this known method is that control of the direction of the drill bit is inaccurate because it relies on rotation of the whole of the drill pipe which can result in unpredictable degrees of rotation of the drill bit. Furthermore in some applications such as with the use of continuous drill pipe it is not practical to rotate the drill pipe.